Diesel-cycle engines have displaced Otto-cycle internal combustion engines in medium and heavy truck use and are becoming increasingly popular for passenger vehicles. The diesel is inherently more efficient but its lack of responsiveness, its noise and its distinctive odor historically limited its appeal to commercial trucks, traveling salesmen and taxicab fleets. The advent of laws and regulations addressing emissions from “mobile sources” also limited the appeal of diesel cars and light tracks because they were perceived as dirty and regional regulations relating to emissions, especially soot, limited their availability. The Otto-cycle engines were easier to modify to comply with more stringent emissions requirements in part because there are more adjustable parameters and soot is not an issue. Recent advances in fuel injection management in combination with electronic systems have closed the gap but the strategies applied to Otto-cycle engines do not always work with diesels.
Oxygenates such as MTBE and ethanol are frequently blended into gasoline to meet air pollution regulations. Ethanol is preferred because it is a renewable resource, is less toxic and politically popular since it is home-grown and is an added market for farmers (through subsidies). Ethanol is readily blended into gasoline, but is difficult to blend into diesel fuel which has a blend of thousands of paraffinic, naphthalenic and aromatic hydrocarbons ranging in carbon numbers between 10 and 22.
Control of emissions from heavy duty diesel trucks and urban buses has become more stringent in recent years and will become more stringent in 2004, when emissions of oxides of nitrogen (NOx) must be reduced to 2.0 g/bhp-hr and in 2007 when NOx will be reduced to 0.2 g/bhp-hr. To achieve the latter levels and to allow for improved particulate matter (soot) traps and NOx catalysts, ultra low sulphur fuel (15 ppm) will be phased-in in 2006. Alternative fuel blends and efficient catalysts will be required.
WO 93/24593 discloses a stabilized, auto-igniting alcohol-containing fuel for use as a diesel fuel having 20 to 70% by volume lower order alcohol (ethanol), 30 to 80% by volume diesel fuel, 4.5 to 5.5% by volume higher order alcohol surfactant, 1 to 15% of a tertiary alkyl peroxide, 3% alkyl peroxide and 0.05 to 0.1% by volume of an anti-clogging additive.
U.S. Pat. No. 6,068,670 discloses an emulsified fuel including water which is more stable than that disclosed in French patent application serial number 2 470 153 which included water and ethanol and was deemed to be unstable on storage.
U.S. Pat. Nos. 6,190,427 and 6,306,184 disclose an E-diesel fuel which is believed to be a product commercially available at this time. The fuel contains 3 to 18% ethanol, 6.5 to 10% of a stabilizer (ethoxylated fatty alcohols), and the remainder commercial No. 2 diesel oil. Optionally, an alkyl ester of a fatty acid and a cosolvent may be added.
U.S. published patent application 20020104256 is directed to the addition of oxygenates to ultra low sulphur automotive diesel oil (ULSADO), the form which will be required by 2006. Accepting the presumption that oxygenates reduce the production of particulates (soot), the reference discloses the use of oxygenates which are saturated, monohydric alcohols having 4 to 20 carbon atoms.
WO 02/059236 discloses compositions to stabilize hydrocarbon fuel over a range of alcohol and water concentrations as an emulsion and includes three different non-ionic surfactants. Optionally, a cetane improver may be employed.
A persistent problem for lean-burn engines such as diesel engines has been production of oxides of nitrogen, typically a mixture of NO and NO2 most frequently referred to as NOx. While oxides of sulphur can be reduced by using ULSADO, the primary source of NOx is nitrogen in the air and the higher temperatures of lean burn engines exacerbates an already known problem. Catalysts will be required.
Methods are known to reduce NOx to N2 and H2O. The most established methods use ammonia, isocyanic acid or precursors such as urea. Representative examples are U.S. Pat. Nos. 6,203,770; 6,066,303; and 4,403,473 and published patent application 20020152745.
Ammonia is a viable and affordable method for controlling NOx at fixed sources but is impractical for mobile sources, especially mid-sized and compact cars. The proven methods require introduction of the reductant upstream of the reducing catalyst, require separate storage of fuel and reductant, thereby requiring inter alia, separate fueling streams.
An alternative reducing system uses hydrocarbons as the reductant. The hydrocarbon may be separate from the diesel fuel as disclosed in U.S. Pat. No. 6,006,515 and in SAE Paper No. 2000-01-2823, or a slip stream from the fuel. The disadvantage of such a system is that fuel economy is impacted and the fuel must be carefully metered to avoid hydrocarbon emissions. This approach results in relatively low NOx conversion compared with other methods
A second alternative is the use of ethanol as a reductant. According to U.S. Pat. Nos. 6,030,590; 6,045,765; 6,057,257; 6,129,713 and 6,284,211 as well as SAE paper 2001-01-1935, the ethanol is introduced between the engine exhaust valve and the catalyst, which is taught to be a silver-based compound. The Caterpillar Inc. DeNOx catalyst system, available since 1996, uses such a protocol. The use of ethanol, a liquid, is less difficult to utilize than ammonia systems, but still requires a separate tank and a duel fueling capacity at fueling stations.
There remains a need to develop an efficient system for NOx control in a single ULSADO fuel.